1. Field of the Invention
This invention relates to a highly elastic and soft flocked foam which presents a pattern apparently having great depth and which has very good handle. More particularly, this invention relates to flocked foam having a sharp embossed pattern provided by a high-frequency welder and which consists of ridges and recesses of different colors (different hue) or of the same color but with different color depth (different tone of the same hue).
2. Description of the Prior Art
Artificial suede leather imitating natural buck skin and flocked goods having a handle and appearance made to resemble such artificial leather has been gaining acceptance in various applications such as garments, furniture and wall coverings. Various products have been developed, such as a soft flocked foam which comprises a soft substrate foam composed of plastic or rubber which is flocked with short synthetic fibers, typically made of rayon or nylon, to combine the softness of the substrate with the touch of the flock (also referred to as pile fibers) to provide good handle. Virtually none of the currently available rubber foams are ideal as a substrate for flocking. For example, soft polyvinyl chloride (PVC) and soft polyurethane foams commonly available in the market do permit flocking, but they have a number of defects when they are used as substrates for flocking.
The soft foam of PVC is produced by foaming a plasticizer-containing paste which can withstand a temperature of only about 70.degree. C., and when such an adhesive for flocking is heat-cured or baked, the foam softens and fuses. To prevent this, an adhesive that can be heat-cured at a relatively low temperature, and which hence permits the use of low baking temperature is used (e.g., at 125.degree. C. for 10 minutes; the soft PVC foam is generally lined with a layer of cloth that is sufficiently heat-resistant to withstand the baking temperature). But this method does not successfully provide a flocked product having satisfactory resistance to rubbing. While such a soft PVC foam substrate can be flocked in such a manner that a pattern comprising high ridges and deep recesses is provided, the platicizer oozes out of the substrate to cause various undesirable results, such as reduced bond strength between the flock and the substrate, toxicity of the plasticizer and the flocked product becoming hard at low temperature. The reduced strength of the bond between the flocks and the substrate causes many flocks to come off entirely or from the edges of ridges during an embossing operation. In addition, the smallness of the degree to which the soft PVC foam can expand often results in cracking occurring at the edges of ridges. For these reasons, such soft PVC foam can only provide an embossed flocked product of low quality which has no commercial value.
The soft polyurethane foam is generally an open-cellular foam which is composed of a resin having a three-dimensional structure, which therefore precludes embossing with heat. Furthermore, due to the absence of a skin, the binder enters the open cells and it is substantially at the cell walls that the foam is flocked, providing a flocked product that is low in fastness to rubbing. For these reasons, soft polyurethane foam is generally not capable of providing a flocked product suitable for embossing.
Other soft foams include a sponge of vulcanized rubber. But vulcanized rubber is intrinsically a heat-set product and is not amenable to embossing with heat. Besides, it is not used in a continuous sheet. Therefore, vulcanized rubber is not suitable for use as a substrate for a flocked foam product of this invention.
A foam is generally provided with an embossed pattern by passing it through hot rolls, pressing it with a hot press, or by treating it with a high-frequency welder. Hot rolls cannot provide a sharp, intricate and fine pattern. Using a hot press requires heating to a high temperature and cooling before demolding; hence, the cycle, consisting of setting the foam in a mold, heating it under pressure, and then cooling and demolding, takes an extended period of time, making quick embossing impossible. The use of a high-frequency welder is advantageous over these methods, in that it can quickly provide a sharp fine pattern in relief at a low temperature.
Embossing with a high-frequency welder makes use of the following principles: When an insulating material having polar groups in the molecule thereof is placed in an electric field of high-frequency, a periodic current flows to force the polar molecules into motion, such as rotation or vibration, thereby generating heat of friction between molecules. This means a work piece generates heat within itself, which enables even a thick-walled article to be heated quickly and uniformly throughout the depth of the article. The amount of heat generated varies with the type of work piece, the shape of the electrodes, the spacing between electrodes, the high-frequency voltage used, and the frequency used.
In using a high-frequency welder, a mold engraved with a desired pattern is fixed between electrodes contained in two parallel press plates, a work piece, such as a flocked foam, is placed so as to contact the engraved side of the mold, the press plates are closed and oscillated at high frequency for a short period of time (generally a few seconds), and the mold is removed to leave a foam having an embossed pattern on the flocked surface. Loss of the embossed pattern (i.e., return to original shape) can be prevented by cooling the mold to a temperature slightly lower than the softening point of the substrate resin of the foam before it is demolded. The usual method to shorten the overall welding cycle is to maintain the parallel press plates at a temperature slightly lower than the softening point of the foam substrate resin. In other words, the temperature elevated by high-frequency heating is reduced by cooling. Thus, embossing with a high-frequency welder uses the heat generated by the motion of the molecules of the resin, and that heat plasticizes the resin by elevating its temperature to higher than the softening point, at which the resin can be processed into a desired shape. It is therefore necessary that the resin be thermoplastic. A prerequisite for embossing is that the resin softens to a degree sufficient to work it into a desired shape (insufficient softening causes the loss of the pattern obtained in relief). The term "welding" as used herein means that adjacent portions of two or more thermoplastic resins are melted and bonded together by applying pressure. Thus, "welding with a high-frequency welder" as used herein means that the thermoplastic resins are melted by heating with a high-frequency welder, and then pressure is applied thereto to bond the thermoplastic resins. In this regard, the joined portion of the termoplastic resins are melt-bonded and unified completely. In welding thermoplastic resins, the resin must have a melting point such that it can be welded to another object because this phenomenon is necessary for the melting of the resin with heat and bonding to another object in the fused portion. The term "embossing" as used herein means that a pattern of ridges and recesses is provided to a foam substrate (or foam sheet); that is, the foam substrate is softened by heating and the softened foam cells are compressed and broken by applying a pressure and thereby a desired pattern of ridges and recesses is set on the foam substrate.
Most of the soft rubber foams and soft plastic foams that are conventionally used as substrates for flocked foam are not amenable to welding or embossing with a high-frequency welder. Of the two soft aforementioned foams, only the soft PVC foam can be welded or embossed using a high-frequency welder (but as also mentioned before, a commercially acceptable flocked PVC foam has not been produced thereby). The foam of 1,2-polybutadiene resin or a polymer blend containing such a resin as one component thereof generally precludes welding with a high-frequency welder, but it can be embossed with the machine. The reason is the 1,2-polybutadiene resin or the polymer blend containing such a resin as one component thereof is partially cross-linked before foaming, and the resulting foam retains some degree of thermoplasticity. Therefore, when heated, it softens but does not melt completely. Thus, it can be embossed but cannot be easily welded with a high-frequency welder. This property can advantageously be used for providing a flocked foam with an intricate pattern having more than one different level. If the resin is of such a nature that it can be welded, all stamped portion of the resin (i.e., a recess) melt to lose shape and the very small difference in the levels between a ridge and a recess is lost. In addition, the foam loses elasticity and becomes rigid.
As mentioned already, general purpose soft polyurethane foam with a dense three-dimensional structure does not soften and hence, it cannot be embossed.
Even if the foam substrate for flocking is amenable to embossing, the formation of an embossed pattern having well defined ridges and recesses is impossible if the flock materials to be impregnated in the substrate are not amenable to embossing. For example, rayon fibers, which do not soften to fuse under heating, or nylon fibers having high softening point (e.g., nylon stable softening at 180.degree. C. or higher, and the filament of nylon 66, which softens at 230.degree. to 235.degree. C.) or a high melting point (e.g., the staple of nylon 6 melts at 215.degree. to 220.degree. C., and the filament of nylon 66 at 250.degree. to 260.degree. C.). The term "soften to fuse" as used herein means softening but not melting (i.e., heating to a temperature range of between higher than a softening point and lower than a melting point), and does not include the heating of the resin and/or the flock material to a temperature higher than its melting point that would cause welding of the resin and/or the flock material. If they are embossed at higher than their melting points, the resin and/or flock material solidify and become rigid upon cooling after the melting. This results in a product which is hard only in the recesses and has its flocked (i.e., sueded) surface impaired.